Clarify squashing of pull request commits

This commit rewords the note regarding the squashing of pull request
commits to make it clearer that the note applies to everyone and not
just external contributors.

CONTRIBUTING.md

@@ -79,7 +79,8 @@ reviewers. If you need to pick up changes from master, you can merge
 master into your branch. A reviewer might ask you to rebase a
 long-running pull request in which case force pushing is okay for that
 request. Note that squashing at the end of the review process should
-also not be done, we can do that if necessary.
+also not be done, that can be done when the pull request is [integrated
+via GitHub](https://github.com/blog/2141-squash-your-commits).
 
 Contributing to the Elasticsearch codebase
 ------------------------------------------